Trepanning is a procedure for providing a hole by cutting along a circular path, and removing the core surrounded by the path after the cutting operation has been completed. The practice is very old, and is used frequently in machining large-diameter holes, where it is more economical to machine away only the material along the circular path, rather than all the material within the hole. The procedure is also useful where it is desired to preserve the interior material.
After a hole has been trepanned, it is usually necessary to bore the hole to closer tolerance and finish than trepanning tools provided. The standard procedure in such cases would be to remove the trepanning tool and replace it with a boring bar of completely different design. In boring deep holes, the machinist must either use a starting bar to pre-bore the workpiece to a specific diameter and depth to engage the pilot of the final boring bar, or set up a support bushing to guide the bar as it enters the trepanned hole. The replacement of the trepanning unit with a boring bar obviously requires additional alignment and securing operations as the tools are successively placed in operating condition on the machine.